Improving the Electrochemical Performance of Glucose-Derived Hard-Carbon Na-Ion Battery Anodes By Silicon Oxycarbide Embedment

Sodium-ion batteries (NIB) hold much promise for energy storage application due to the high abundance and low cost of sodium, combined with the low redox potential of Na/Na⁺ of -2.71 V vs. SHE, which is only 0.3 V above the one of Li/Li⁺. However, sodium-ion batteries are less explored and research for promising electrode materials is still a fundamental concern [1].

Within this work, we present a novel approach to increase the electrochemical performance of glucose-derived hard-carbon (HC-G) NIB-anodes by the preparation of silicon oxycarbide-based composites. In particular, the influence of an intrinsically carbon-rich (SiOC/HC-G) and carbon-poor (SiOC(N)/HC-G) ceramic as stabilizing matrix phase is discussed.

Materials were prepared by mixing of glucose with commercially available polysiloxane or polysilazane polymers, followed by thermolysis at 1000 °C. XRD identified the samples of amorphous nature, whereas Raman-Spectroscopy emphasized the disordered character of the generated hard-carbon phase. Compared to pure HC-G, both composites reveal higher insertion/extraction capacities (Figure 1). In addition, SiOC(N)/HC-G shows an excellent cycling stability, while the cycling stability of SiOC/HC-G is less. However, much higher capacities are obtained for the SiOC/HC-G sample. Moreover, SiOC/HC-G demonstrates the best coulombic efficiency of the first cycle of about 83 %, whereas the first cycle efficiency of pure HC-G only amounts 19 %; for SiOC(N)/HC-G 51 % are obtained.

In both cases, the glucose reacts with the polymer precursor upon thermal treatment, yielding a homogeneous composite after pyrolysis. The ceramic matrix acts as laminate towards the interspersed hard carbon and stabilizes the graphene layers against exfoliation during the subsequent insertion and extraction of sodium cations. In the case of SiOC/HC-G, the SiOC phase provides additional free carbon, accounting for the higher capacities. However the laminating effect is less, as compared to SiOC(N)/HC-G, which leads to the weaker cycling stability.

[1] S.-W. Kim, D.-H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Enery. Mater., 2 (2012) 710-721.

Caption to Figure 1: Galvanostatic cycling performance of glucose-derived hard-carbon (HC-G) and SiOC-based/hard-carbon composites SiOC/HC-G and SiOC(N)/HC-G; current rate 37 mA/g.